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> All the other languages I know of, like this proposal for Java, require the variable to be redefined. can you give a few examples? This makes no sense to me

• gizmo686 4y ago
  Consider the following pseudo-Java: Object foo = ... if (foo instanceof String) { System.out.println(foo.toLowerCase()) } The above is not valid Java, because foo has type Object, which does not have a toLowerCase() method, so the compiler refuses to compile it. What you need to do is something like: Object foo = ... if (foo instanceof String) { String sfoo = (String) foo System.out.println(sfoo.toLowerCase()) } Granted, you don't necessarily need to explicitly redefine in, as Java allows anonymous casting: Object foo = ... if (foo instanceof String) { System.out.println(((String) foo).toLowerCase()) } Regardless, the point still stands. Java (and most strongly typed languages with an inheritance based type system) requires an explicit cast, even if a straight forward static analysis would show that the object in question is already the desired type.
  • GavinBierman 4y ago
    This is all true, but with pattern matching for instanceof you don't need the explicit cast: Object foo = ... if (foo instanceof String s) { System.out.println(s.toLowerCase()) } In this case, the scope of the pattern variable `s` is just the then block, so are free to reuse it elsewhere in the block.